Coating process for ferrous metal surfaces



United States Patent 3,533,859 COATING PROCESS FOR FERROUS METAL SURFACES Rudolf Engesser and Richard Tuch, Frankfurt am Main,

Werner Rausch, Stierstadt, Taunus, and Winfried Menzer, Sprendlingen-Hirschsprung, Germany, assignors to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York No Drawing. Filed June 13, 1967, Ser. No. 645.633 Claims priority, application Germany, June 18, 1966,

Int. Cl. C23f 7/10 US. Cl. 148-615 6 Claims ABSTRACT OF THE DISCLOSURE A process for forming a protective and/or paint-base coating on ferrous metal surfaces wherein the surface to be treated is contacted with an aqueous acidic zinc phosphate solution, substantially free of fluoride, which solution contains at least one oxidizing agent accelerator, boric acid, and has a total P 0 content within the range of about 2 to 12 grams per liter. The ratio of free P 0 to the total P 0 in the phosphatizing solution is maintained at a value of from about 0.032 to 0.280, the specific value for this ratio within this range being dependent upon the solution temperature and the total P 0 content of the solution. The preferred oxidizing agent accelerators in the solution are nitrate and nitrite and the solutions desirably also contain a titanium phosphate and a non-ionic wetting agent.

This invention relates to an improved process for coating ferrous metal surfaces and more particularly relates to an improved method of forming thin, hard, tightly adherent zinc phosphate coatings on ferrous metal surfaces, which coatings have excellent corrosion resistant and paintbase properties.

Zinc phosphate solutions have been extensively used in the chemical surface treatment of ferrous metals, such as iron and steel, as a preparation for the subsequent application of a paint or lacquer. The zinc phosphate layers produced on the metal surfaces by these processes are known to increase the corrosion resistance and improve the adhesion of paint or lacquer films which are applied. Thin, hard zinc phosphate layers have been found to be particularly suitable, since, with these, excellent adhesion values are obtained even where the painted part is subjected to bending stresses.

In order to obtain the desired thin phosphate layers, various modifications of the typical phosphatizing solutions are required. Thus, it is known that the addition of small amounts of a polyphosphate to a nitrate accelerated zinc phosphate bath will result in appreciable reductions in the coating weight of the phosphate layer which is produced. Difficulties are encountered, however, in baths of this type inasmuch as rapid analytical methods for making a quantitative determination of the polyphosphates are not generally available. Thus, in replenishing baths of this type, the amount of polyphosphate added is based, to a large extent, on previous operating experiences, rather than on actual analytical determinations. This may create problems, inasmuch as the presence of an excess amount of the condensed phosphate is found to lead rapidly to a suppression of the formation of the desired crystalline layers and a coating is obtained whose corrosion protective action is greatly inferior.

It is also known that the coating weights of zinc phosphatizing solutions can be reduced by adding large amounts of calcium to zinc phosphate baths which contain oxidizing agents. To be effective, however, the calcium must be present in an amount which is comparable to the 3,533,859 Patented Oct. 13, 1970 ice zinc concentration of the bath. In determining the pointage of such a bath, this large amount of calcium contributes appreciably to the sodium hydroxide consumption in the titration, thus indicating an incorrect high value for the zinc content. Accordingly, when replenishing such a processing bath to maintain a constant pointage, the ratio of zinc to calcium in the replenishing material may be disproportionately low. This results in a shift in the weight proportions of zinc to calcium in the processing solution in favor of the calcium so that ultimately, deficient or even no zinc phosphate layers are formed.

It is further known, particularly with respect to the coating of aluminum and aluminum alloys, that hard thin, thin coating layers may be obtained when using a hot aqueous coating solution containing monoplrosphate, an oxidizing agent, fluoroborate, and an excess of boric acid. Such processing solutions are disadvantageous, however, in that the fluoride containing materials require special handling procedures and, additionally, the fluoride containing waste from such process requires special disposal processes.

While it is also known that good paint base coatings may be attained with iron oxide-iron phosphate layers, deposited from aqueous solutions of alkali phosphate, the corrosion resistance of such coatings is frequently appreciably less than that of the crystalline zinc phosphate coating. Thus, in many instances, such coatings are not satisfactory.

It is, therefore, an object of the present invention to provide an improved process for forming thin, tight, hard zinc phosphate coatings on ferrous surfaces.

A further object of the present invention is to provide an improved process for forming zinc phosphate coatings on ferrous metal surfaces, which coatings have excellent corrosion resistance and paint-base properties.

These and other objects will become apparent to those skilled in the art from the description of the invention which follows.

Pursuant to the above objects, the present invention includes a process for producing a thin, hard phosphate coating on ferrous metal surfaces which comprises contacting the ferrous metal surface to be treated with a coating composition comprising an aqueous acidic zinc phosphate solution, substantially free of fluoride, which solution contains at least one oxidizing agent accelerator, boric acid, and has a total P 0 content within the range of about 2 to 12 grams per liter, adjusting the weight ratio of free P 0 to total P 0 in the solution interpendent upon the solution temperature and total P 0 content within the following ranges:

Weight ratio range of free P20 to total P20 in the bath at Total P205,

grams/liter 40 C. 50 C. 60 C. 70 C.

and maintaining the weight ratio of free P 0 to total P 0 within these ranges while contacting the ferrous metal surface to be treated with the solution for a period sufiicient to form the desired coating. By operating in this 1 of fluoride. As is known in the art, phosphatizing baths of this general type contain the zinc ions in the form of zinc dihydrogen phosphate and desirably have a pH Within the range of about 2.0 to 3.5.

Various oxidizing agent accelerators may be utilized in these processing solutions such as nitrates, nitrites, combinations of nitrates and nitrites, chlorates, bromates and the like, as are known to those in the art. Generally, these oxidizing agent accelerators are present in amounts up to about 2% by weight of the phosphatizing solution with amounts within the range of about .01 to 1% being preferred. In many instances, the preferred oxidizing agent accelerator is a combined nitrate-nitrite accelerator and reference will be made to phosphatizing solutions containing such accelerators. Typically, the nitrate ions are present in amounts within the range of about 0.05 to 2% by weight of the phosphatizing solution while the nitrite ions are present in amounts within the range of 0.001 to 0.05% by weight of the solution. It will be appreciated, however, that, as are known to those in the art, amounts outside of these typical ranges may also be used.

The boric acid component of the phosphatizing compositions of the present invention is desirably present in an amount of at least 0.5 gram per liter of H BO with greater amounts, up to the maximum solubility of the boric acid in the solution being suitable. The boric acid may be introduced into the bath solution as such, i.e., as H BO or it may be added as various boron containing compounds which will form boric acid in the aqueous acidic phosphatizing solution. Exemplary of such boron containing compounds which may be used are alkali metal borates, such as Na B O .10H O and NaBO .5H O. The boric acid or boric acid forming compound may be added directly to the phosphatizing bath or they may be added to the concentrate compositions which are used in making up and replenishing the phosphatizing bath and added to the bath in this manner.

In addition to the above components, it has also been found to be desirable if the phosphatizing solutions of the present invention contain, as an additional component, an activating-acting titanium phosphate. These activatingacting phosphates are customarily used in aqueous solution, as a pretreatment for metal surfaces before the application of a phosphate coating. Typically, they are compositions prepared from disodium orthophosphate and a soluble titanium compound, such as titanyl sulfate and processes for their preparation are set forth in German Pats. 1,144,565 and 85,638. Desirably, the activating-acting titanium phosphates are incorporated in the phosphatizing baths in amounts of at least about 3 milligrams per liter of the phosphatizing solution with amounts up to as much as about one gram per liter being typical. It is to be appreciated, of course, that in some instances, amounts of the activating-acting titanium phosphate in excess of these typical amounts may also be used with satisfactory results.

It has been found that by the incorporation of this titanium phosphate material in the phosphatizing bath, the rate at which the coating layer is formed is increased. Moreover, it is found that an improvement in the coating itself is obtained which is in excess of that obtained by the individual components of the phosphatizing bath, independent of their concentrations in the solution. Finally, it is found that a high degree of uniformity of the phosphate coating is obtained when the activating-acting titanium phosphates are included in the composition, as the phosphate coating is formed uniformly on the metal surface even in those areas which contain traces of dirt,

action is imparted to the phosphatizing solution, particularly when the solution is applied by spray. Desirably, the wetting agent is present in the solution in an amount within the range of about 0.05 to about 1 gram per liter of the phosphatizing solution. Although various non-ionic wetting agents may be used, the preferable wetting agents have been found to be the ethoxylated alkyl phenols and straight chain fatty alcohols, particularly those having an est-erified terminal OH group in the ethylene oxide chain, which latter wetting agents are characterized by their low foaming properties.

As has been noted hereinabove, an important part of the operation of the process of the present invention is the maintenance of the ratio of free P 0 to total P 0 in accordance with the temperature of the phosphatizing solution and its total P 0 content. The phosphatizing solutions for use in the present process have a total P 0 content within the range of about 2 to 12 grams per liter and the ratio of the free P 0 to the total P 0 is adjusted and maintained within the following ranges:

Total P205, grams/liter When operated in this manner, the total P 0 content of the phosphatizing solution corresponds approximately to the following total point values:

Total P 0 Approximate total grams per liter: point value 2.5 5.5 5 O 11.0 7 5 16.5 10 0 22.0

The total point values, of course, are the number of milliliters of a 0.1 normal sodium hydroxide solution which is required to titrate a 10 milliliter sample of this phosphatizing solution to the phenol-phthalein end point.

In determining the free P 0 content of the phos phatizing solution, this may be done by titrating a 10 milliliter sample of the phosphatizing solution with 0.1 normal sodium hydroxide using dimethyl yellow as the indicator. In this titration, one milliliter of the 0.1 normal sodium hydroxide solution corresponds to 7.1 milligram of free P 0 in the phosphatizing solution. The total P 0 in the solution may be determined using any of the various known analytical methods, including titration, precipitation, or colorimetric analysis. For example, the phosphatizing solution may be titrated to form a magnesium phosphate precipitate, which precipitate may then be ignited and weighed to determine the total P 0 or the precipitate may then be titrated with EDTA to determine the P 0 content' In another analytical method, ammonium molybdate may be added to a sample of the phosphatizing solution, forming a yellow ammonium phosphomolybdate precipitate, which precipitate may then be titrated with sodium hydroxide to determine the P 0 content. In a colorimetric determination, ammonium molybdate and ammonium vanadate may be added to the phosphatizing solution, followed by the addition of nitric acid, and the resulting solution compared colorimetrically to standards containing a known amount of P205- Once the determination of the free P 0 and total P 0 content of the solution has been made, the adjustment of the ratio within the desired range, depending upon the bath temperature and the total P 0 content thereof, may be made in any convenient manner. Gen erally, it has been found desirable to make this adjustment by adding sodium hydroxide to the phosphatizing solution until the desired ratio is obtained. Other adjustments of the phosphatizing solution to obtain the desired ratio 5 of free P total P 0 as are known to those in the art, may also be utilized.

The phosphatizing solutions of the present invention may be formulated using any convenient source of materials which provide the desired components in the bath. Exemplary of suitable materials are zinc oxide, phosphoric acid, zinc dihydrogen phosphate, zinc nitrate and the like. Additionally, the nitrite ions, which are accelerators in the bath, may conveniently be added as the alkali metal nitrite, such as sodium nitrite. Moreover, as has been indicated hereinabove, the boric acid may be added either as such, or as a boron containing compound, such as an alkali metal borate, which will form boric acid in the solution. While other materials may be used to formulate these phosphatizing solutions, as are known to those in the art, it is to be appreciated that in choosing these materials, the materials used are desirably those which will not introduce extraneous ions into the treating solution, or, that at least will not introduce ions into the solution which are detrimental either to the solution itself or to the coating which is produced.

Desirably, the phosphatizing solutions are at a pH within the range of about 2.0 to about 3.5 and are used at a temperature within the range of about 40 to 70 degrees centigrade, although operations outside of these ranges may also be carried out in some instances. The phosphatizing solutions may be applied using any suitable application techniques, including immersion, flooding,

spraying, and the like, although the advantages of the present process are particularly apparent when using flooding and spraying methods.

The phosphatizing solutions, as described above, are brought into contact with the ferrous metal surface to be treated, i.e., an iron or steel surface, using a suitable application technique. The metal surface is maintained in contact with the phosphatizing solution for a period suflicient to effect the formation of the desired phosphate coating on the metal. Typical contact times which may be used are within the range of about 1 to 4 minutes, although contact times outside of this typical range will often be used in many instances, depending upon the particular application technique which are employed. After the desired phosphate coating has been formed on the metal surface, the surface may then be given a final rinse with a trivalent or hexavalent chromiumcontaining solution, aqueous solutions containing from about 0.01 to about 1% by weight of CrO either alone or in admixture with other acids, such as phosphoric acid, being typical of the rinse solutions which may be used. The thus-treated metal surfaces may then be given a protective coating of a paint or lacquer, which paint or lacquer coatings may be applied by conventional dip, spray, or flooding techniques or by electrophoretic means, the conditions of such application techniques being known to those in the art. The zinc phosphate coatings which are thus-produced on the ferrous metal surfaces treated are found to have a very fine-grain crystalline structure and are dark in color. Typically, the weights of the coatings are within the range of about 1 to 2 grams per square meter.

It is found that with the phosphatizing solutions described above, which contain the non-ionic wetting agent, the process of the present invention may often be carried out in fewer treating stages because of the combined degreasing and dirt removing action with the phosphatizing action of the solution. Thus, in the first treating zone, the metal surfaces are both degreased and phosphatized, rinsed with water in the second zone and then given a final rinse or treatment with the aqueous acid solution of hexavalent chromium or trivalent chromium, in the third zone. In many instances, because of the excellent corrosion protection provided by the coatings which are produced in accordance with the operation of the method of the present invention, particularly when they are used in conjunction with paint or lacquer coat- .ings, it is possible to eliminate the final after rinse with the hexavalent or trivalent chromium solution without any appreciable reduction in the corrosion protection which is obtained.

It has also been found that in using the phosphatizing solutions which have been described, substantially all of the grease and dirt removed from the metal surfaces is retained in the phosphate sludge which form-s in the coating solution. Thus, the solution containers and the walls in the phosphatizing zone remain substantially free of grease-containing deposits and the grease-containing phosphate sludge is easily removed from the coating bath by filtration. This combined degreasing and phosphatizing also results in far less incrustation of spray nozzles in the phosphatizing bath, as well as of the heat recorder, bath and tunnel walls, as compared to that obtained in a sequential degreasing and phosphatizing process. Additionally, with this combined degreasing and phosphatizing action, any prepassivation of the metal surfaces is minimized. In contrast, this prepassivation is frequently encountered in a sequential degreasing-phosphatizing process as a result of the initial reaction of the phosphatizing solution mist with the cleaned work surface as it is introduced into the phosphatizing zone, which reaction results in the formation of thin tarnish layers on the metal surface, which hinder the formation of a homogeneous zinc phosphate coating during the phosphatizing step.

In order that those skilled in the art may better understand the present invention and the manner in which it may be practiced, the following specific examples are given. In these examples, unless otherwise indicated, temperatures are in degrees centigrade and parts and percent are by weight. It is to be appreciated, however, that these examples are merely exemplary of the present invention and are not to be taken as a limitation thereof.

EXAMPLE 1 An aqueous phosphatizing solution was prepared which contained the following components in the amounts indicated:

This solution was then modified by the addition of the additive as set forth in the following table and the ratio of free P 0 to total P 0 was adjusted by the addition of sodium hydroxide to the solution to the values shown in the table. Steel plates, degreased in an alkaline spray cleaner were rinsed with water and then sprayed for three minutes with the solutions which were at a temperature of 60 degrees centigrade. The coating weights obtained in each instance are given in the table. It is to be noted that the activating-acting titanium phosphate, which was added to some of the solutions, was a mixture of of Na HPO and 10% of titanium phosphate.

Coating F P O weight, ree 2 rams Solution Additive and amount total P20 Sq metei a None 0.12 2. 2-2. 4 b 4 grams/liter Na B O,-. 10H2O. 0.12 1. 3-1. 5 c None- 0. 08 2. 6-2. 8 d 4 grams/liter N82B40r.10H20 0.08 2. 4-2. 6 e. 0.2 gram/liter activating-acting-ti- 0. 12 2. 2-2. 3

tanium phosphate. f 0.2 gram/liter activating acting-titanium phosphate and 4 grams] liter Na2B40r, 10H2O. g 0.5 grain/liter non-ionic wetting O. 12 2. 2-2. 4

agen h 4 grams/liter Na2B4O1. 10H2O 0.5 0. 12 1 1-1. 3

gram/liter non-ionic wetting agent and 0.2 gram/liter activating-acting titanium phosphate. i None 0.19 4grams/literNazB4O1.10H2O 0. 19

1 Translucent layer.

From the above results, it is seen that only with solutions b, and h, which solutions are the only ones which contain the boric acid and have the ratio of free P to total P 0 "within the ranges specified hereinabove, for the total P 0 content of the bath and the solution temperature used, in accordance with the process of the present invention, is the desired thin, hard phosphate coating produced on the metal surface.

EXAMPLE 2 Undegreased but rust and scale free plates of carbon steel were treated as follows:

In a first treating zone, the plates were sprayed for four minutes at a temperature of 60 degrees centigrade with a solution 2.55 grams per liter N0 0.15 gram per liter NaNO 4 grams per liter N32P407-1OHgO, 0.2 gram per liter of a mixture containing titanium phosphate and 90% Na HPO and 0.5 gram per liter of a non-ionic wetting agent. Additionally suflicient sodium hydroxide was added to the solution to obtain a ratio of free P 0 to total P 0 of about 0.12.

In a second treating zone, the steel plates from the zone were sprayed with cold water.

In a third treating zone the steel plates from the second zone were sprayed for 30 seconds at a temperature of 40 degrees centigrade with an aqueous solution containing 100 milligrams per liter CrO and were then finally sprayed with deionized water and then were dried.

The treating solution used in zone 1 was continuously replenished with a phosphate concentrate containing 12.42% zinc, 27.14% P 0 18.59% N0 and the balance water, to a constant point total of 15. Additionally, the treating solution was also continuously replenished with a mixture containing 74% NaNO 10% Na B O .10H O, 7.5% of a nonionic wetting agent, 1% of titanium phosphate and 7.5% of Na HPO to maintain an NaNO content in the treating solution in the range of 0.15 to 0.20 gram per liter. The process was continued until four square meters of steel plate were treated. During this time, the ratio of free P 0 to total P 0 was Within the range of 0.11 to 0.13 and the H BO content of the bath was within the range of 1.2 to 1.8 grams per liter. The coatings produced on the steel plate thus-treated were deep dark gray in color, uniform, having extremely fine crystalline structure and were completely grease-free. The weights of these coatings were within the range of 1.2 to 1.8 grams per liter.

EXAMPLE 3 Steel plates which had been treated with solution a through h of Example 1 and those which had been treated in accordance with the procedure of Example 2 were electrophoretically coated with a lacquer while other plates from the same example were treated with a mono layer acrylate lacquer. All of the thus-treated plates were then tested by bending over a conical mandrel and also in the salt spray test using the procedure ASTM B 117- 54T. The plates treated with solutions b, and h of Example 1 and in accordance with the procedure of Example 2, showed no breaking or scaling 011 of the lacquer at a bend mandrel diameter of 3.3 millimeters, while the plates treated with the remaining solutions of Example 1 showed appreciable breaking and loosening of the lacquer film after bending over mandrel diameters within the range of 10 to 30 millimeters. In the salt spray test, after 48 hours, the electrophoretically lacquered plates which had been treated with the solutions b, f, and h of Example 1 and in accordance with the procedure of Example 2 were substantially free of any migration under the coating at the scratch site while on the plates treated with the remaining solutions of Example 1 the lacquer was loosened at distances of from 2 to 3 millimeters from the scratch site. With the plates coated with acrylate resin lacquer, after 96 hours in the salt spray test, the plates treated with solutions 12, f and h of Example 1 and in accordance with the procedure of Example 2 showed migrations at the scratch site of from only from 0 to 0.5 millimeter while the plates treated with the remaining solutions of Example 1 showed migrations of from 1 to 1.5 millimeters at the scratch site.

While there have been described various embodiments of the invention, the compositions and methods described are not intended to be understood as limiting the scope of the invention as it is realized that changes therein are possible and it is intended that each element recited in any of the following claims is to be understood and referring to all equivalent elements for accomplishing substantially the same results in substantially the same or equivalent manner, it being intended to cover the invention broadly in whatever form its principle may be utilized.

What is claimed is:

1. A process for forming a thin, hard phosphate coating on ferrous metal surfaces which comprises contacting the ferrous metal surface to be treated with a coating composition consisting essentially of an aqueous acidic zinc phosphate solution, substantially free of fluoride, which solution contains at least one oxidizing agent accelerator, at least 0.5 gram per liter boric acid, and has a total P 0 content within the range of about 2 to 12 grams per liter, adjusting the weight ratio of free P 0 to total P 0 in the solution, interpendent upon the solution temperature and total P 0 content, within the following ranges:

Total P10 grams/liter 40 C. 50 0. 60 C. 70 C.-

and mantaining the weight ratio of free P 0 to total P 0 within these ranges while contacting the ferrous metal surface to be treated with the solution for a period sufficient to form the desired coating.

2. The process as claimed in claim 1 wherein the oxidizing agent accelerator is a combination of nitrate and nitrite ions.

3. The method as claimed in claim 2 wherein the solution also contains at least three milligrams per liter of an activating-acting titanium phosphate.

4. The method as claimed in claim 3 wherein the solution also contains from about 0.05 to 1 gram per liter of a non-ionic wetting agent.

5. The method as claimed in claim 4 wherein the solution has a pH within the range of about 2.0 to 3.5.

6. The method as claimed in claim 5 wherein the contact of the treating solution in the ferrous metal surface is effected by spraying the solution on the metal surface.

References Cited UNITED STATES PATENTS 1,610,362 12/1926 Coslett 1486.15 X 2,346,302 4/1944 Hays et al. 1486.15 X 2,479,564 8/1949 Gilbert 148-6.15 2,783,208 2/1957 Katz 1486.15 X 2,884,351 4/195'9 Cavanagh et al 1486.15 3,090,709 5/1963 Henricks 1486.15 2,322,349 6/1943 Jernstedt 1486.15 3,420,715 1/1969 Ayres 1486.15 X

RALPH S. KENDALL, Primary Examiner 

